An electrode stack is constructed by stacking an intended number of strip-like positive electrodes including a positive electrode lead and strip-like negative electrodes including a negative electrode and sandwiching separators made of a strip-like microporous polyethylene film between the positive and negative electrodes, the separators including substantially the same shape and size as the strip-like negative electrodes. To prevent lithium precipitation from edges of the strip-like negative electrodes during repeated charging and discharging, the outer circumference of the positive electrode is sized to be smaller than the outer circumference of the negative electrode. Here, the outer circumference of the positive electrode corresponds to the inner circumference of the negative electrode such that the positive electrode is placed inside the electrode surface of the negative electrode.
However, in the case of stacking the strip-like positive and negative electrodes, a lithium ion battery has a problem in which stacking misalignment between the positive and negative electrodes may occur since the positive electrode is smaller than the negative electrode and a portion serving as a reference for stacking the positive electrode on the negative electrode substantially does not exist.
In the case of the stacking misalignment, edges of the positive and negative electrodes are aligned in part, or the electrode surface of the positive electrode protrudes out of the electrode surface of the negative electrode. In addition, some problems that greatly degrade the reliability of the battery may occur. For example, lithium precipitates from electrode edges due to repeated charging/discharging, cycle characteristics degrade, and short-circuits occur in the battery. Then, additional processes are required to test or correct stacking misalignment, and thus the manufacturing process becomes complicated.
To overcome such problems, Japanese Patent Laid-Open No. 07-302616 discloses a square type lithium ion secondary battery including a bag-like separator and a strip-like positive electrode located inside the separator. This document discloses a type in which the bag-like separator is a strip-like microporous plastic film, with at least one thereto-bonding section formed in each of at least two adjacent sides thereof, and the strip-like positive electrode is located by the them-bonding sections. This document also discloses another type in which at least one thermo-bonding section is formed on each of four sides of the bag-like separator, and the strip-like positive electrode is positioned by the thereto-bonding sections.